1. Field of the Invention
The present invention relates to a belt member such as a transfer material conveying belt, an intermediate transfer belt, and the like, and a so-called electrophotographic belt, which are used in an electrophotographic apparatus, and further, it relates to the production method of the electrophotographic belt, and the electrophotographic apparatus having the electrophotographic belt.
2. Related Background Art
In recent years, a so-called electrophotographic belt such as a transfer material conveying belt, an intermediate transfer belt, and the like has often come into use for a color electrophotographic apparatus. These electrophotographic belts are usually stretched across by two or more rollers, and are installed within the electrophotographic apparatus, and are driven by at least a roller (driving roller) of these rollers.
The color electrophotographic apparatus using the electrophotographic belt is classified broadly into the following two types.
A first type, as shown in FIGS. 1 and 2, is a so-called tandem type color electrophotographic apparatus in which different color toner images formed respectively on the surfaces of plural electrophotographic photosensitive members are transferred in order on the transfer material or the intermediate transfer belt conveyed by the transfer material conveying belt.
A second type, as shown in FIG. 3, is a so-called four pass type color electrophotographic apparatus using an electrophotographic photosensitive member and an intermediate transfer belt, and after the intermediate transfer belt makes four turns, collectively transferring on the transfer material.
Nowadays, since the color electrophotographic apparatus has been solicited to improve an image output speed, the tandem type color electrophotographic apparatus has been on the increase, which is advantageous in speeding up among the color electrophotographic apparatuses.
Against such a background, as one of the characteristics required for the transfer material conveying belt and the intermediate transfer belt, stability of the peripheral speed is cited. If the peripheral speed is not stable, different color toner images cannot be superposed at a desired position of the transfer material on the transfer material conveying belt or the intermediate transfer belt, thereby causing a color shift. Although some variable factors of the peripheral speed are believed to exist, as a cause arising from the electrophotographic belt itself, unevenness of its thickness can be cited.
In the four pass type color electrophotographic apparatus, the intermediate transfer belt is allowed to make one turn in order to transfer the next color. Consequently, fluctuation of the peripheral speed generated by unevenness of the thickness of the intermediate transfer belt and the color shift generated by fluctuation of the peripheral speed are cancelled in theory. Accordingly, the four pass type color electrophotographic apparatus rather than the tandem type color electrophotographic apparatus is advantageous in controlling the color shift. Naturally, even if the four pass type color electrophotographic apparatus is advantageous, in reality, it is not that the color shift does not disappear at all, but that the color shift generated by fluctuation of the peripheral speed occurs more or less.
In contrast to this, in the case of the tandem type color electrophotographic apparatus, since the transfer of the next color is performed before the transfer material conveying belt or the intermediate transfer belt makes one turn, the effect on the color shift by the thickness of these belts is great. Hence, the electrophotographic belt such as the transfer material conveying belt and the intermediate transfer belt used for the tandem type color electrophotographic apparatus is required to have much fewer unevenness in thickness.
Further, at present, regardless of the tandem type color electrophotographic apparatus and the four pass type color electrophotographic apparatus, since the demand for price-reduction is remarkable, the electrophotographic belt much lower in price has been solicited.
The electrophotographic belt (transfer material conveying belt and the intermediate transfer belt) used currently on the market can be classified broadly into three types in terms of materials and production processes.
A first type is a so-called thermosetting resin belt in which resin (precursor) before hardening is added with conductive agent and the like, and after that, is solidified by cure reaction by heating. As its representative example, for example, a belt using polyimide resin is cited. This is disclosed in Japanese Patent Application Laid-Open No. 2001-064389 (Patent Document 1).
The thermosetting resin belt is often produced by a centrifugal compacting method, in which coating material becoming the material of the belt is coated inside a mold, and the coating material is uniformly extended within the mold by the centrifugal force. Hence, the acquired belt has an advantage of being excellent in thickness uniformity. Consequently, in general, this type of the belt is believed to be advantageous in color shift.
However, there has been a problem in that it is only immediately after starting using the belt that the color shift of this type of the belt is very few, and as the belt is repeatedly used, the color shift becomes worse. Further, since the thermosetting resin belt takes a long stretch of time in heat curing and evaporation of solvent at the production time, it is not suited to a lower cost production.
A second type is a so-called rubber belt produced in such a manner that unvulcanized rubber is added with the conductive agent and the like, and after that, is vulcanized and grinded. The rubber belt can be made excellent in durability by weaving a core of fabric into rubber.
However, since vulcanization and grinding require a long stretch of time, the rubber belt is not suited to a lower cost production. Further, since the rubber belt is prone to be elastically deformed at the rotational driving time, and easily generate microscopic fluctuation in peripheral speed, the color shift is easily generated.
A third type is a so-called thermoplastic resin belt obtained in such a manner that thermoplastic resin composition added with the conductive agent and the like on thermoplastic resin is extruded into a tube shape, and is cut off in a predetermined length. Since the thermoplastic resin belt can be produced by continuous extrusion at a lower cost, it is most advantageous in keeping the cost down.
However, since it is difficult to completely uniformize a gap in the outlet (die lip) of an annular die used when extruding the thermoplastic resin composition in its peripheral direction, there is a problem in that unevenness of the thickness (polarized thickness) is prone to be generated.
Further, with respect to the production of the belt by extrusion, various investigations have been conducted so far.
For example, in Japanese Patent Publication No. 2886350 (Patent Document 2), a method is disclosed in which the tube extruded from the annular die is brought into contact with a heat regulated mandrel, and at the same time, is sprayed with heat regulated gas. Since the tub extruded by this method is produced by contacting the mandrel, microscopic protrusions of the inner peripheral surface are crushed and made flat, and the irregularity of thickness becomes within ±5%.
However, according to the researches conducted by the present inventors, it has been found very difficult to remove the unevenness of thickness in the peripheral direction. From among the belts prepared by the above described method, the belt having the smallest thickness was selected, and when the color shift arisen from the usage was confirmed, though the initial color shift was not so much developed, deterioration of the color shift due to repeated usage was worse.
Further, in Japanese Patent Application Laid-Open No. H11-344025 (Patent Document 3), it is disclosed that, if the temperature of the annular die is changed in the peripheral direction, the belt having the unevenness of thickness controlled within ±5% can be obtained.
However, according to the researches conducted by the present inventors, even by this technology, the unevenness of thickness has been unable to be sufficiently reduced. The present inventors presume that this is due to the fact that the annular die is usually made of metal. That is, since metal is good in heat conduction, in case the temperature of the annular die is changed in the peripheral direction, heat is also transferred to the heated periphery. As a result, temperature distribution of the peripheral direction in the annular die becomes broadened, and therefore, removal of microscopic polarized thickness becomes difficult. Eventually, the obtained belt has been a belt that creates a great color shift.
Further, in Japanese Patent Application Laid-Open No. H11-344025, it is disclosed that plural places of the extruded tube are sprayed with air different in quantity and pressure, thereby obtaining a belt with the unevenness of thickness controlled within ±5%.
However, according to the researches conducted by the present inventors, there is a space between adjacent nozzles, and in that space, no correction effect of thickness can be obtained. Hence, it has been not possible to sufficiently reduce the unevenness of thickness. If the number of nozzles is increased, the space is made smaller, but, since an air content for one each nozzle is reduced, in the space between the nozzles, it has been not possible to obtain the thickness correction effect after all, and it has been not possible to sufficiently reduce the unevenness of the thickness. The obtained belt has been a belt that creates a great color shift.